1. Field of the Invention
The present invention relates to a stencil printer, and more particularly, to a multi-drum type stencil printer equipped with a plurality of printing drums.
2. Description of the Prior Art
As it gets more popular to produce multi-color overlaid prints by the stencil printing, various sorts of multi-drum type rotary stencil printers equipped with a plurality of printing drums have been tried to make it possible to produce multi-color stencil prints by one printer. In the overlaid prints it is necessary that a plurality of different printings are successively carried out by exactly maintaining the relative position between the respective prints. Because of this requirement, in many of such multi-drum type rotary stencil printers equipped with a plurality of printing drums, a basic construction is such that the plurality of printing drums are rotationally driven around respective central axes by common drive means in synchronization with one another.
On the other hand, each printing drum of the multi-drum type rotary stencil printers incorporates therein an inner press roller contacting the perforated circumferential wall thereof at the inside thereof so as to support the perforated circumferential wall at its inside, while supplying ink to the perforated circumferential wall from the inside thereof, and a doctor rod arranged close to the inner press roller in parallel thereto, so as to define an ink depositing groove together with an outer circumferential surface of the inner press roller, the ink depositing groove having a wedge-shaped cross section for holding an ink deposit therein. Further, in such a construction, the inner press roller is often adapted to be rotationally driven in synchronization with the printing drum by a transmission of rotation of the printing drum thereto via rotation transmitting means such as gear wheels.
When a plurality of printing drums having the above-mentioned basic construction are driven by the common driving means, it is convenient that a plurality of gear trains are arranged so as to mesh respectively with a common drive member such as a gear wheel, and the respective printing drums are driven by the respective gear trains. By such an arrangement, the plurality of printing drums can be driven by the common driving means in synchronization with one another.
When a multi-drum type rotary stencil printer of the above-mentioned basic construction is operated so as to produce prints each bearing overlaid prints supplied thereon by the operation of all of the plurality of printing drums, in each of the printing drums the ink of the ink deposit is successively consumed according to the progress of printing. However, if one of the printing drums is not operated while other one or more of the printing drums are operated, since the ink of the ink deposit of the non-operated printing drum is also drawn out from the ink deposit as carried on the surface of the inner press roller according to the rotation thereof due to a synchronized idling rotation of the non-operated printing drum, with the drawn out ink being returned to the ink deposit without being transferred to the print sheets, it soon occurs that the ink of the ink deposit is too much stirred and causes an undesirable change of its quality.
In contemplation of such a problem of undesirable change of quality of the ink anticipated in the multi-drum type rotary stencil printers of the above-mentioned basic construction, it is an object of the present invention to provide a multi-drum type rotary stencil printer improved so as to avoid such a problem.
According to the present invention, the above-mentioned object is accomplished by a multi-drum type rotary stencil printer comprising a plurality of printing drums each having a perforated circumferential wall and incorporating therein an inner press roller contacting the perforated circumferential wall at the inside thereof so as to support the perforated circumferential wall from the inside thereof, while supplying ink to the perforated circumferential wall from the inside thereof, and a doctor rod arranged adjacent the inner press roller in parallel thereto so as to define an ink depositing groove together with an outer circumferential surface of the inner press roller, the ink depositing groove having a wedge-shaped cross section for holding an ink deposit, the inner press roller being adapted to rotate in synchronization with the printing drum by a transmittance of rotation of the printing drum via rotation transmitting means, the plurality of printing drums each being adapted to be rotationally driven around each central axis in synchronization with one another by common drive means via each drive linkage, wherein a clutch is provided for selectively interrupting the drive linkage between the common drive means and at least one of the plurality of printing drums.
By such a clutch being incorporated between the common driving means and at least one of the plurality of printing drums which is selectively not operated during the operation of the other printing drum or drums of the multi-drum type rotary stencil printer wherein the plurality of printing drums are adapted to be rotationally driven by the common driving means around the respective central axes in synchronization with one another, the one printing drum is held not to rotate in the meantime by the clutch, so that the inner press roller of the one printing drum is not idly rotated, whereby any excessive stirring of the ink deposit in the wedge-shaped ink depositing groove formed between the inner press roller and the doctor rod is avoided.
In the multi-drum type rotary stencil printer of the above-mentioned construction, the common drive means may comprise a first gear wheel adapted to rotate integrally with the press roller, while each of the printing drums may comprise a second gear wheel adapted to rotate integrally therewith, and the drive linkage for each of the printing drums may comprise a gear train including a third gear wheel meshing with the first gear wheel and a fourth gear wheel meshing with the second gear wheel, the clutch being incorporated between the third and fourth gear wheels. By such an arrangement, the drive linkage for each of the printing drums is available in a compact construction requiring only one additional axis for rotary members.
In the above-mentioned construction of the drive linkage, the drive linkage may comprise a planetary gear mechanism having a ring gear adapted to rotate integrally with the third gear wheel, a sun gear adapted to rotate coaxially with the fourth gear wheel, a carrier and a plurality of planetary pinions supported by the carrier to be rotatable about respective axes thereof and meshing with the ring gear and the sun gear, the carrier rotatably supporting the planetary pinions being finely adjustable of its angular position around a coinciding central axis of the ring gear and the sun gear, with the clutch being incorporated between the sun gear and the fourth gear wheel. By such an arrangement, the rotational position of each of the printing drums relative to the press roller can be finely adjusted for a fine adjustment of the longitudinal position of the print image produced by each of the printing drums on a print sheet, as required for the overlaid printing by the plurality of printing drums.
In the above-mentioned construction of the drive linkage, the clutch may comprise a spline bore formed in a hub of the sun gear, a spline shaft axially slidably engaged in a spline bore formed axially through the fourth gear wheel and selectively engageable in the spline bore of the sun gear, and means for selectively shifting the spline shaft in an axial direction thereof so that the spline shaft is engaged into or disengaged from the spline bore of the sun gear. By such an arrangement, the clutch is conveniently incorporated in the one additional axis for the rotary members.
Further, the second gear wheel may be provided as a pair arranged at opposite axial ends of the printing drum, while the fourth gear wheel may also be provided as a pair to mesh with the pair of second gear wheels, the drive linkage further comprising a hollow shaft connecting the pair of fourth gear wheels integrally with one another, with an axial extension of the spline shaft being passed through the hollow shaft so as to be operated at an end thereof remote from the spline bore of the third gear wheel for the selective engagement therewith. By such an arrangement, the printing drum is rotationally driven from the common driving means via the drive linkage with no twisting being applied thereto.